1. Field of the Intention
The present intention relates to a composite material turbine wheel and, more particularly, to a turbine wheel in which a hub contains a drive gear composed of teeth extending radially from the one wheel's faces.
The intention applies more specifically to turbine wheels made of ceramic matrix type of composite material, such as used in gas turbines.
2. Prior Art
Thermostructural composite materials are noted for having mechanical properties that make them amenable for use in structural elements that face up to large stresses, and for being able to maintain those properties at high temperatures. These materials are composed of a fiber-based reinforcement and a matrix that densifies this reinforcement. The material forming the fiber reinforcement is either carbon or a ceramic, such as silicon carbide. The matrix material is either carbon (as in carbon-carbon composites), or a ceramic (as in a ceramic matrix composite material, otherwise known as CMC).
A CMC turbine wheel can be made from a fibrous preform consisting of plies of a two-dimensional texture, such as a carbon or ceramic cloth. The plies are piled on each other and may be linked together (e.g. by needling or thread implants). The preform is densified by the matrix material through a gaseous or liquid process. In the first case, the matrix is formed by chemical vapor infiltration using known methods. In the second case, the matrix is formed by impregnating a matrix precursor and transforming the precursor by a chemical process that will yield the matrix material.
After densification, the turbine wheel is machined to bring it down to its final dimensions and to form its drive gear.
FIGS. 1 and 2 show a wheel 1 made of a CMC type of composite material. The hub 2 has a classical drive gear 3 made of teeth 4 that extend radially from one of the wheel's faces 1. The wheel 1 is driven into rotation by a gearwheel 5 (shown in FIG. 2 only) affixed to a shaft 6 passing through the turbine wheel.
The gear section of wheel 5 is comprised of radially-extending teeth 8 shaped to conform with the teeth 4 with which it intermeshes. For the sake of convenience, the wheels 1 and 5 in FIG. 2 are shown spaced apart from each other along their common axis.
In operation, the turbine wheel 1 is driven into rotation by the gearwheel 5 against which it is forced into contact by tightening devices (not shown).
The pressure with which the gearwheel 5 applies this tightening force against the turbine wheel 1 creates stress concentrations at the roots of the cogs cut in the wheel to form the teeth 4, and induces large interlaminar stresses within the composite material forming the wheel 1. Indeed, as shown in the detailed view of a part of gear sections 3 and 7 depicted in FIG. 3, the tightening force (arrows F) causes reactions on the tooth shoulders along directions parallel to composite material's fiber reinforcement plies, creating the above-mentioned interlaminar stresses that can be a source of delamination (slippage between the plies).